The present invention relates to an electric flight control system for an aircraft, in particular for a transport aircraft, and a method for controlling the flight of an aircraft with the aid of such a system.
In a standard manner, an aircraft flight control system comprises:                at least one mobile control surface, able to be deflected as a function of deflection commands received;        at least one control stick able to be actuated by a pilot of the aircraft, the position of the control stick generating first guidance commands representative of the position;        an automatic piloting device able to generate second guidance commands; and        at least one flight control computer which calculates control surface deflection commands, with the aid of an integrated computation means which contains piloting laws and which uses for this calculation guidance commands, the guidance commands corresponding to the first guidance commands during manual piloting and to the second guidance commands during automatic piloting.        
Moreover, the automatic piloting device generally employs, in certain modes of operation, a protection of the flight domain of the aircraft (high and low speeds, high angle of attack, excessive attitudes) protecting the flight against excessive values of speeds and attitudes. Moreover, this automatic piloting device is generally limited in authority (for example +/−0.3 g longitudinally and +/−30° laterally).
With such limits, the performance and comfort objectives allotted to the automatic piloting device are attainable. Moreover, because of these limits, the effect of a fault of the automatic piloting device remains restricted, thereby making it possible to simplify the architecture and to lower the criticality of the faults of the automatic piloting device and therefore to simplify the design and validation thereof. Most automatic piloting devices exhibit authority limited to the exact need, and the entire design and fault analysis rest upon this assumption.
Moreover, the electric flight control system allows the crew at any moment to obtain the maximum maneuverability available, while remaining in a protected flight domain (in terms of speed, angle of attack and attitude). If the crew, in manual piloting mode, deflects the control stick fully, the system makes it possible to glean the maximum from the aircraft's usual capabilities.
To cope with exceptional events such as wind gusts or large wind gradients (change of wind direction), a dedicated unit disengages the automatic piloting device when the latter does not succeed in maintaining the aircraft in the normal flight domain, on account of its limited authority. An alarm is then emitted and the crew must optionally carry out compensation by using the full authority available with the manual flight controls.
Although not presenting any safety problem, such a manner of operation induces an overload of work for the crew and a risk of deviation of the initial trajectory, whereas the exterior disturbance phenomena are often momentary, and the flight could, after the event, continue to be carried out with the automatic piloting device if it has remained engaged.
Moreover, the automatic piloting device cannot be engaged when the aircraft is outside of the normal domain of use of the device. In such a situation (when the aircraft is outside of the flight domain in which the automatic piloting device operates normally), the pilot must therefore pilot the aircraft manually to restore it to the normal flight domain. It will be possible to reengage the automatic piloting device only when the aircraft is back in the normal flight domain.
This standard electric flight control system therefore generates an overload of work for the pilot under particular flight conditions, for which the automatic piloting device does not succeed in maintaining the aircraft in the normal flight domain, on account of its limited authority.